Unidirectional rotation of micromotors on water powered by pH-controlled disassembly of chiral molecular crystals

Biological and synthetic molecular motors, fueled by various physical and chemical means, can perform asymmetric linear and rotary motions that are inherently related to their asymmetric shapes. Here, we describe silver-organic micro-complexes of random shapes that exhibit macroscopic unidirectional rotation on water surface through the asymmetric release of cinchonine or cinchonidine chiral molecules from their crystallites asymmetrically adsorbed on the complex surfaces. Computational modeling indicates that the motor rotation is driven by a pH-controlled asymmetric jet-like Coulombic ejection of chiral molecules upon their protonation in water. The motor is capable of towing very large cargo, and its rotation can be accelerated by adding reducing agents to the water.

Optical images were taken using an Olympus BX51 microscope, both with and without a polarization filter. SEM images were done using FESEM or JSM-6300 instruments. TEM investigation was carried out using a 200 kV JEOL JEM 2011, FasTEM, and a JEOL JEM-2100F TEM operating at 200 kV equipped with JED-2300T EDX.

XPS
XPS measurements were performed on a Kratos AXIS-Ultra DLD spectrometer, using a monochromatic Al k source at a power ranging between 15-75 W and detection pass energies of 20-80 eV. The pressure in the analysis chamber was kept below 10 -9 torr. Ar-ion sputtering was used only at fine levels, such as to follow the very early stages of removal of adsorbed molecules from the highly corrugated surface. Marked differential charging was typically encountered at the platelet aggregates, which normally complicates the interpretation of XPS results. Here, however, differential charging was exploited to differentiate between signals originated in different domains, which enhances the capabilities of the (large area) XPS probe significantly. 1,2 Samples for the XPS analyses were prepared by carefully lifting Si wafer substrates that were pre-positioned at the bottom of the reaction cell. Thus, platelets floating on the liquid surface could be captured at various stages of the spinning process. To selectively measure the molecules released from the platelets to the water, we removed all observable aggregates from the liquid surface prior to lifting the Si-wafer up. Macroscopic fragments of the Langmuir film could thus be XPS-analyzed. (1,2) Surface pressure.
The surface pressure was measured using a Wilhelmy plate attached to a sensor from Nima Technologies. Due to the small difference of surface pressure, pressure was measured using a Petri dish (3.5 cm diameter), at a rate of 2 samples per second. The surface of the water was first cleaned using a vacuum aspirator to remove dust and other possible materials from the surface.
Samples of chimots or Camphor particles were measured from the moment they were placed on the surface.
XRD XRD scans spectra taken by with a Bruker D8 DISCOVER diffractometer using CuK radiation.

XPS results
XPS measurements of the platelet aggregates showed the characteristic signals of silver in a nonmetallic state, together with the carbon and nitrogen of cinchona alkaloid molecules. Signals from exposed areas of the Si substrate were differentiated by means of controlled surface charging (CSC) (1,2), exploiting the marked difference in electrical conductivity between the platelets and the substrate. We could thus estimate quite accurately the amount of oxygen and carbon at the aggregates, despite the interfering substrate-related signals.
Two peaks in the N 1s line, with roughly equal intensities and a binding energy difference of 2.5-2.8 eV, were found to be a characteristic fingerprint of the bare molecules. Independently, the C/N concentration ratio (after subtraction of related substrate signals) was evaluated, yielding values close to (yet slightly higher than) the theoretical value, which reflects the presence of small amounts of CH-based adsorbents on top of the Ag-cin complex. The aggregate-related oxygen signals yielded a O/N ratio greater than 1, while the bare molecular atomic ratio should be 1. Finally, the Ag 3d line did not exhibit any hint of plasmonic satellites, suggesting that none of the XPS-available regimes can be considered metallic. The latter observation agrees with the elevated charging encountered, as well as with the low Ag atomic ratios, see Supplementary   Table 3.
In the following, we describe the main variations encountered under different sample treatments and preparation procedures. The N/Ag atomic concentration ratio is given in Supplementary Table 2, providing a measure of the amount of adsorbed molecules. It shows a clear decrease in the amount of platelets after spinning, compared to their amount prior to spinning. Notably, after spinning in BH 4 , the amount of cin molecules is even smaller than those detected after spinning in water, in agreement with our direct observations and the faster kinetics achieved upon BH 4 addition to the water.
Analysis of those Si-substrates that were lifted up (after pre-removal of the floating Ag-platelets) provided direct proof for the formation of a well-defined film (believed to be Langmuir-like) by those molecules released during platelets spinning: All expected characteristics of the cinchona alkaloids were found to appear in these films, whereas the amount of silver was practically zero.
Similar results were obtained in measurements near (off) platelets in the regular samples, yielding vanishing Ag signals.
A fine Ar-sputtering would normally be enough to remove all adsorbed molecules in thin organic coatings. However, such a result was not achieved in the case of chimots. This result is attributed to the rich morphology of the studied particles and the potentially large portion of shadowed areas that could not be sputtered effectively. Yet, based on the very early steps of sputtering (up to 2 min at a flux equivalent to a sputtering rate of ~1A/sec in Ta 2 O 5 ), we could clearly identify the initial removal of cinchona alkaloids (see N/Ag in Supplementary Table 1). Subsequent sputtering steps reached saturation in the N/Ag ratio, suggesting that the removal of cinchona alkaloids in the non-shadowed areas is instantaneous. Additionally, the same behavior was observed after spinning, indicating that even after the spinning was terminated, many molecules remained in the Ag aggregate. This result agrees with our experiments demonstrating that the chimots resume their spinning motion upon replacement of the solvent with fresh water. Table 2: 1. The N/Ag concentration ratio:

Detailed features are indicated by Supplementary
a. Before spinningthe same value is obtained for cin+ and cin-, in agreement with expectations.
b. After spinning in water (until the motion was terminated for the first time) -N/Ag decreases, indicating the release of molecules during spinning, but also that the release is not complete.
c. Bottom side of the plates -N/Ag gives even lower values, suggesting that the release takes place at the water-platelet interface. Note that most of our samples consisted of a random combination of top vs bottom faces, in contrast to this dedicated experiment. d. After spinning in BH 4a larger decrease is observed in N/Ag when compared to spinning in water, which correlates well with the direct observations of quicker spinning in BH 4 . Note that the decrease is more moderate for cin-compared to cin+. Together with point b above, it may reflect a lower release-efficiency for cin-. However, the statistical validity of this conclusion cannot be stated confidently due to the lack of data points for cin-, which were all taken from the same propelling experiment. e. After sputtering -N/Ag decreases, as expected from molecules coating a silvercontaining substrate/core. Two additional comments should be noted in this respect: i. Saturation was quickly reached, such that continued sputtering did not quite affect it. The reason for that is that the surfaces of the samples are very rough, with at least 50% of the area shadowed from the ion beam. Such morphology offers huge surface area available for the spinning mechanism, a feature supported by other findings as well.
ii. The value of N/Ag before sputtering is too large to be associated with a thin layer coating on a flat surface. Extensive roughness needs to be considered, with b. After spinning in waterthe C/N ratio increases. This is consistent with a mechanism where the cin+ and cin-molecules are released as a whole, while potential hydrocarbon contaminants are not as readily released into the water.
This conclusion is further confirmed by the 'bottom' sides, which more readily release molecules; hence the C/N ratio is found to be even higher.
c. After sputtering -A small decrease is obtained, as expected from the removal of the entire molecule. Note that sputtering of the C-H moieties should give rise to more pronounced changes in the ratio of total C/N. d. After spinning in NaBH 4 -Reliable values were unable to be attained due to uncertainties as with the differential charging analysis (the CSC). e. Langmuir film -Interestingly, data in these cases gave binary values: either O/N=1 or 2.
4. N1/N2: For cases with a high release (low N/Ag) we find increased N-reduction vs Noxide (details not provided in the table).
5. Ag Auger -The Auger line of silver (not included in the table) did give indications for an increased metallic component after spinning, especially in the NaBH 4 solvent.

Simulation
In the absence of noise, one may use the following generalized Stokes' law for modeling the hydrodynamics of chimots. The effective force (F) and torque (T) acting on the particle are related to its translational velocity ( ) and angular velocity (ω) via the grand-resistance (dissipation) matrix (H) 3 : , where η denotes the dynamic viscosity of the fluid.
The above formulation holds, providing the characteristic Reynolds number is small. The We employ here a two-dimensional self-propulsion model, similar to that reported by Kuemmel 4 for studying the motion of micron-sized L-shaped particles made of light-sensitive material. In this case, one gets: [2] where φ describes the orientation of the force in the lab's frame of reference and θ represents the angle between the force and the translational velocity v . To simplify the formulation, we enforce the following symmetries in the grand-resistance matrix, namely: H yy =H xx , H xy =H yx =0, H xφ =H φx =H yφ =H φy . In turn, the corresponding parameters in the grand-resistance matrix can be accordingly expressed in terms of a typical linear size r of the particle as: H xx = β xx r, H xφ = β xφ r 2 , and H φφ = β φφ r 3 . Note that for a freely suspended spherical particle of radius r in the Stokes regime, β xx = 6π, β xφ = 0 and β φφ = 8π, but for our geometries, these factors take other numerical values. In this study, we select r as the radius of an equivalent circle with an area equal to that of the parallel projection of the particle on a plane above it and parallel to the water surface. For the first particle, we get r = 0.32 mm, and for the second particle, r = 0.62 mm. The resulting equations render an analytical solution given by: which is subjected to the initial condition .
Once φ is known, the speed v can be readily computed as: , and the angle of the velocity vector can be directly determined from .
It is not easy to analytically determine the behavior of φ, v, and θ from these equations, but an asymptotic analysis in terms of a small parameter β xφ /β xx , renders (to leading-order) the following expressions: , , .
The expansion of the linear velocity provides a simple way for fitting the experimental results of Fig. 3(f) to our model. The period τ in the experimental results can be identified as τ = ηβ φφ r 3 2π/T, its average velocity v = F/(ηβ xx r), and the difference between its maximum and minimum by ∆v = 2√2(β xφ /β xx ) (T/ηβ φφ r 2 ). In other words, given the experimental results for η, r, τ, v , and ∆v, we can specify the parameters governing our model as T/β φφ = 2πηr 3 /τ, F/β xx = ηrv , and β xφ /β xx = ∆vτ/(4π√2r). The quotient β xx /β φφ affects the distance traveled by the particle during one period (τ) and is adjusted here to fit the experimental results. We could try to use the MD simulation results to provide a rough estimate of the driving forces acting on a single chimot to justify the forces used in the above hydrodynamic model. The calculated energies presented in Fig. 3b show that the (-1 0 0) and (1 0 0) charged facets release molecules to the surrounding water, with a slight difference in their release rates (roughly 10%).
For simplicity, consider that the surface tension of water drops to 50% of its original value of γ facet = 36 nN/μm (γ water = 72 nN/μm) when fully saturated with amphiphilic cin molecules. We can also assume that the surface coverage above a particular facet is roughly proportional to its ejection rate of the cin molecules. This rate is proportional to the difference in binding energies to water and the crystal, ΔE facet . Consequently, the surface tension above a particular facet can be estimated from γ facet = γ water -c·(γ water -γ saturated )·ΔE facet , where c accounts for the diffusion (removal) of released molecules. If the transport of released molecules is fast, but there is still a 10% difference in release rates between the facets, we can estimate the differences in surface tension across the crystal to Δγ crystal = 3.6 nN/μm. From ΔE facet = γ facet ·L·Δx, we can estimate the energy change of a liquid surface of γ facet that is in contact with the facet of a length L and that moves in the orthogonal direction by Δx. The force acting on this facet, F facet = ΔE facet / Δx = L .
γ facet , produces a total force of ΔF crystal = L· Δγ crystal = 9 nN, which is acting on a crystal with the side length of L = 2.5 μm, that is exposed to the surface tension difference of Δγ crystal = 3.

Supplementary Figures
Supplementary Figure 1. Three hours recording of chimots' velocities. Most chimots terminate their motion within 20 seconds. Few chimots continue rotating between 1000-7000 seconds.

Supplementary Tables
Supplementary  Table 3. XPS data recorded from samples before spinning and after spinning in water or BH4. Selected atomic concentration ratios (C/N, O/N and N/Ag) are given, corresponding to the aggregate-related components only. Data after moderate sputtering are provided for two representative cases. The label 'Bottom' stands for samples where Ag aggregates were carefully turned up-side-down, such as to selectively measure the face that was (during spinning) directly exposed to the water. Langmuir stands for samples lifted after removal of the Ag platelets from the water surface, such as to selectively measure the Langmuir film of molecules released from the platelets. After placement onto the surface of water, organic molecules appear to be lost from the chimot to the surrounding water. However, silver does not appear to be ejected from the chimots. Addition of NaBH4 results in faster loss of organic molecules from the chimots. * Aggregate-related components only